Optical non-reciprocal devices, for example isolators, are arranged to pass a light beam in a forward direction but not allow a reverse beam, such as might be caused by unwanted reflections, to pass in the reverse direction. Typically, the devices achieve this non-reciprocal function by means of optical elements which displace the reverse light beam away from the axis of the input fiber.
One example of such devices is disclosed by K. W. Chang et al in an article entitled "High-performance single-mode fiber polarization-independent isolators", OPTICS LETTERS, Apr. 15, 1990, Vol. 15, No. 8, Optical Society of America, the entire contents of which are incorporated herein by reference. It includes two collimating lenses, one for expanding the light beam from an input fiber and the other for re-focussing the light onto the output fiber. A non-reciprocal isolator is located between the collimating lenses. The isolator comprises Faraday rotators and birefringent "walk-off" crystals. The arrangement is such that forward-travelling light beams are displaced in a "positive" direction and reach the output fiber whereas backward-travelling light beams are displaced in the "negative" direction and do not reach the input fiber. In this structure, however, the backward-travelling beam must be shifted by more than the width of the expanded beam to achieve good isolation. Consequently, a longer birefringent "walk-off" crystal must be used. For example, with a beam diameter of 0.5 mm., the length of birefringent crystal should be about 5 mm. This results not only in a large device but also higher cost.
A different non-reciprocal optical isolator, which dispenses with the collimating lenses and hence can be smaller, has been disclosed by K. Shiraishi et al in an article entitled "Polarization-Independent In-Line Optical Isolator with Lens-Free Configuration", Journal of Lightwave Technology, Vol. 10, No. 12, December 1992, the entire contents of which are incorporated herein by reference. Shiraishi et al use an array of thermally expanded core (TEC) fibers, i.e. which have been heated to cause their cores to expand. An isolator chip is positioned in a slot cut through the expanded portions of the fibers dividing them into input fibers and output fibers. The isolator chip comprises a sandwich of birefringent plates and Faraday rotators--without collimating lenses. The ends of the input and output fibers abut the isolator chip. Because the beam diameter from the fibers is relatively small (dependent upon the mode-field diameter of the fibers), the required isolation can be achieved with a relatively small beam shift, resulting from the use of thinner birefringent plate. The device is not entirely satisfactory, however, because the light beam is neither collimated nor guided as it passes between input and output fibers, resulting in higher loss due to diffraction and greater difficulty in achieving low back reflection with low insertion loss.
U.S. Pat. No. 4,375,910 (Seki) issued Mar. 8, 1983, the entire content of which is incorporated herein by reference, discloses an optical isolator comprising a Faraday rotator for 45.degree. rotation of polarization, a pair of lenses arranged on opposite sides of the Faraday rotator and a pair of birefringent crystal plates on the outer sides of the respective lenses and with their "principal planes" displaced 45.degree. from each other. Seki claims that placing the birefringent crystal plates where the light beams are constricted reduces the amount of offset needed to prevent the reflected light from returning to the source, thereby allowing thinner birefringent crystal plates to be used. Seki also states that the use of two lens elements reduces coupling loss. Nevertheless, the device is not entirely satisfactory because it would be relatively expensive to make and cannot readily be adapted to form a non-reciprocal device other than an isolator.